This invention concerns a method of plating aluminum workpieces in the presence of ultrasonic agitation and more specifically has reference to a method wherein an aluminum workpiece is disposed in a body of molten metal maintained at an elevated temperature while the molten metal is subjected to sonic or ultrasonic energy. Exemplary of such a method is the placing of an aluminum workpiece into an ultrasonic apparatus comprising a pool of molten tin which is maintained at an elevated temperature and which is subjected to ultrasonic energy produced by electroacoustic or magnetostrictive transducers either coupled to the outside surface of the tank supporting the pool or by means of sonically activated horns contacting the pool.
As is known in the prior art, tinning of aluminum using tin or lead-tin alloys can be done most effectively by means of an ultrasonically activated bath. The ultrasonic energy displaces the tenacious oxide layer normally adhering to the aluminum surface and causes a clean oxide-free surface to which the tin adheres.
The above described process has received renewed attention in connection with the desire of producing parts, especially electrical conductors, made of aluminum rather than copper. Aluminum is not only less expensive, but is also more plentiful than copper ore.
Heretofore tinned electrical grade aluminum parts, such as buss bar or electrical wire, coated with pure tin or an alloy of lead-tin, have been afflicted with certain shortcomings which have caused the industry to reject the use of such tinned aluminum parts. In particular, when the coating was applied in the presence of ultrasonic activation, the coating failed to provide a strong metallurgical interface bond between the tin and the aluminum. The lack of such bond permitted continued oxidation of the aluminum surface resulting in the occurrence of cracks and even the flaking off of the tin coating.
One prior solution was to use a zinc aluminum (95/5) solder coating on the aluminum part. The solder coating, while forming a metallurgical bond with the aluminum part, also increased the electrical resistance of the finished product. As a result, the zinc-aluminum coated aluminum part has not been successful in many applications, for example as an electrical buss bar termination.
Another application comprises the coating of aluminum horns (also known as resonators, tools, mechanical amplitude transformers etc.) as used for transmitting sonic or ultrasonic vibrations from an electroacoustic transducer to a workpiece. In the past, such horns have been made from titanium. The price of this latter metal recently has tripled and, therefore, it has become more economical to utilize aluminum for those applications where the lower yield strength of aluminum as compared with titanium can be tolerated. However, aluminum exhibits a relatively soft surface and it is necessary to provide a harder finish, usually chrome plating, in order to reduce wear. In view of the fact that chrome cannot successfully be deposited, electrically or chemically, upon aluminum, an intermediate metal coating must be provided. Tin performs such a function. Unless there is an extremely tenacious bond between the aluminum and the tin, the high frequency cyclic expansions and contractions to which a horn is subjected cause a disruption of the bond between the tin and the aluminum and as a result, the chrome layer peels and flakes off. The rather expensive horn must then be replaced.